PIC16C745 Datasheet

IC型号：
PIC16C745
PDF描述：
8-Bit CMOS Microcontrollers with USB
文件大小：
4945.69KB
PDF页数：
165页
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PIC16C745/765

7.3

Timer1 Operation in Asynchronous

Counter Mode

TABLE 7-1:

Osc Type

CAPACITOR SELECTION FOR

THE TIMER1 OSCILLATOR

Freq

32 kHz

100 kHz

200 kHz

33 pF

15 pF

33 pF

15 pF

If control bit T1SYNC (T1CON<2>) is set, the external

clock input is not synchronized. The timer continues to

increment asynchronous to the internal phase clocks.

The timer will continue to run during SLEEP and can

generate an interrupt on overflow, which will wake-up

the processor. However, special precautions in soft-

ware are needed to read/write the timer (Section 7.3.1).

In asynchronous counter mode, Timer1 can not be used

as a time-base for capture or compare operations.

7.3.1

READING AND WRITING TIMER1 IN

ASYNCHRONOUS COUNTER MODE

These values are for design guidance only.

Crystals Tested:

32.768 kHz

100 kHz

200 kHz

Epson C-001R32.768K-A

Epson C-2 100.00 KC-P

STD XTL 200.000 kHz

卤

20 PPM

卤

20 PPM

卤

20 PPM

Reading TMR1H or TMR1L while the timer is running

from an external asynchronous clock will guarantee a

valid read (taken care of in hardware). However, the

user should keep in mind that reading the 16-bit timer

in two 8-bit values itself poses certain problems, since

the timer may overflow between the reads.

For writes, it is recommended that the user simply stop

the timer and write the desired values. A write conten-

tion may occur by writing to the timer registers, while

the register is incrementing. This may produce an

unpredictable value in the timer register.

Reading the 16-bit value requires some care. Examples

12-2 and 12-3 in the PICmicro鈩?Mid-Range MCU Fam-

ily Reference Manual (DS33023) show how to read and

write Timer1 when it is running in asynchronous mode.

Note 1:

Higher capacitance increases the stability of

oscillator but also increases the start-up time.

Since each resonator/crystal has its own charac-

teristics, the user should consult the resonator/

crystal manufacturer for appropriate values of

external components.

7.5

Resetting Timer1 using a CCP Trigger

Output

If the CCP1 or CCP2 module is configured in compare

mode to generate a 鈥渟pecial event trigger鈥?/div>

(CCP1M<3:0> =

1011),

this signal will reset Timer1.

Note:

The special event triggers from the CCP1

and CCP2 modules will not set interrupt

flag bit TMR1IF (PIR1<0>).

7.4

Timer1 Oscillator

Timer1 must be configured for either timer or synchro-

nized counter mode to take advantage of this feature. If

Timer1 is running in asynchronous counter mode, this

reset operation may not work.

In the event that a write to Timer1 coincides with a spe-

cial event trigger from CCP1 or CCP2, the write will

take precedence.

In this mode of operation, the CCPRxH:CCPRxL regis-

ter pair effectively becomes the period register for

Timer1.

A crystal oscillator circuit is built-in between pins T1OSI

(input) and T1OSO (amplifier output). It is enabled by

setting control bit T1OSCEN (T1CON<3>). The oscilla-

tor is a low power oscillator rated up to 200 kHz. It will

continue to run during SLEEP. It is primarily intended

for use with a 32 kHz crystal. Table 7-1 shows the

capacitor selection for the Timer1 oscillator.

7.6

Resetting of Timer1 Register Pair

(TMR1H, TMR1L)

TMR1H and TMR1L registers are not reset to 00h on a

POR or any other reset except by the CCP1 and CCP2

special event triggers.

T1CON register is reset to 00h on a Power-on Reset or

a Brown-out Reset, which shuts off the timer and

leaves a 1:1 prescale. In all other resets, the register is

unaffected.

7.7

Timer1 Prescaler

The prescaler counter is cleared on writes to the

TMR1H or TMR1L registers.

漏

1999 Microchip Technology Inc.

Advanced Information

DS41124A-page 47

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